3.2 C: Capacitor
3.2.1 Syntax
Device
Cxxxxxxx n+ n– value
Cxxxxxxx n+ n– expression
Cxxxxxxx n+ n– value
{IC=initial-voltage}
Cxxxxxxx n+ n– model {L=length}
{W=width} {TEMP=temperature}
{IC=initial-voltage}
.capacitor label n+ n– expression
Model (optional)
.model mname C {args}
.model mname TABLE {args}
3.2.2 Purpose
Capacitor, or general charge storage element.
3.2.3 Comments
N+ and n– are the positive and negative element nodes,
respectively. Value is the capacitance in Farads.
The (optional) initial condition is the initial (time = 0) value of
the capacitor voltage (in Volts). Note that the initial conditions
(if any) apply only if the UIC option is specified on the transient command.
You may specify the value in any of these forms:
- A simple value. This is the capacitance in Farads.
- An expression, as described in the behavioral modeling chapter. The
expression can specify the charge as a function of voltage, or the
capacitance as a function of time.
- A model, as described in the behavioral modeling chapter. The
table model describes a table of charge vs. voltage.
- A model, which calculates the capacitance as a function of
length and width, referencing a .model statement of type C. This is compatible with the Spice-3 “semiconductor capacitor”.
3.2.4 C Model statement
A model statement may be used,, with model type C or Cap.
The parameters are:
- CJ = x
- Junction bottom capacitance. (Farads / meter
squared). (Default = 0.)
- CJSW = x
- Junction sidewall capacitance. (Farads /
meter). (Default = 0.)
- DEFW = x
- Default width. (meters). (Default = 1e-6)
- NARROW = x
- Narrowing due to side etching. (meters).
(Default = 0.)
- TC1 = x
- First order temperature coefficient.
(Farads / degree C). (Default = 0.) (Not in Spice.)
- TC2 = x
- Second order temperature coefficient.
(Farads / degree C squared). (Default = 0.) (Not in Spice.)
- TNOM = x
- Parameter measurement temperature.
(degrees C.). (Default = 27.) (Not in Spice.)
Capacitance is computed by the formula:
capacitance = CJ * (L - NARROW) * (W - NARROW)
+ 2 * CJSW * (L + L - 2 * NARROW)
After the nominal value is calculated, it is adjusted for temperature
by the formula:
value *= (1 + TC1 * (T-T0) + TC2 * (T-T0)^2)
3.2.5 Probes
The following probes (Transient, DC, and OP analysis) are available in
addition to those available for all basic elements. See the print command for documentation.
- DT
-
Time step. The internal time step used for this device for numerical
integration. It is not necessarily the same as the global time step.
- TIME
-
Time. The time of the most recent calculation of this device. It is
not necessarily the same as the global time.
- TIMEOLD
-
The time of the previous calculation of this device. It is not
necessarily the same as the global time.
- TIMEFUTURE
-
The latest recommended time for the next sample, as determined by this
device. The actual time will probably be sooner than this.
- DTREQUIRED
-
The required approximate maximum time step, usually based on
truncation error estimation.
- CHARGE
-
The charge stored in this capacitor, in Coulombs.
- Q
-
The same as charge.
- CAPACITANCE
-
The effective capacitance of this device. For a fixed capacitor, it
be its value. For a nonlinear capacitor, it is the effective
capacitance at this time, or ∂ q / ∂ v.
- DCDT
-
The time derivative of capacitance. For a linear capacitor it will be
zero.
- DC
-
The change in capacitance compared to the previous sample. Its
primary use is in debugging models and numerical problems. For a
linear capacitor it will be zero.
- DQDT
-
The time derivative of charge. Hopefully this is the same as current,
but it is calculated a different way and can be used as an accuracy
check.
- DQ
-
The change in charge compared to the previous sample. Its primary use
is in debugging models and numerical problems.